The present invention is directed to communication receivers and, more particularly, to communication receivers having gain control and narrowband interference detection.
The elimination or minimization of narrowband interference such as intermodulation distortion (IMD), is important to the operation of any code-division multiple-access (CDMA) communication system. Narrowband interference reduces the quality of the signals processed by CDMA receivers by, for example, saturating analog-to-digital (A/D) converters used by CDMA receivers.
One source of narrowband interference may be an amplifier stage of a CDMA receiver. Most commonly, IMD is created in a low power, low noise amplifier (LNA) having a non-linear output characteristic at high input levels. The non-linear characteristic of LNAs is conducive to producing IMD when an undesired signal (e.g., a signal that the receiver is not to receive) is present with the desired signal (e.g., a signal that the receiver is to receive). When a desired signal is amplified by the LNA in the presence of an undesired signal, harmonics of the desired and undesired signals interact to create a third, or higher, order IMD products that fall within the frequency band of the receiver.
Generally, one known technique of combating narrowband interference in a CDMA receiver is to attenuate a received signal before the received signal is amplified by the LNA of the CDMA receiver. In particular, an attenuator in the CDMA receiver decreases the energy level of the received disposed signal to a point at which the received signal-to-noise ratio (SNR) is a minimum acceptable level. This attenuation technique is effective because the amplitude of narrowband interference, such as IMD, reduces exponentially with the attenuation of the amplitude of the received signal. Thus, by attenuating the received signal as much as possible (e.g., to a minimum acceptable SNR level) narrowband interference is reduced as much as possible.
While the attenuation technique is effective in reducing narrowband interference, such a technique has the undesirable effect of reducing SNR to a minimum level, regardless of whether narrowband interference is actually present. Accordingly, SNR is always sacrificed, regardless of whether narrowband interference is present. The needless sacrifice of SNR when narrowband interference is not even present is undesirable in communication systems, such as voice, data or other systems that seek to provide maximal clarity and understandability of communication.
Another technique for eliminating narrowband interference includes eliminating the low power LNA of the receiver in favor of a higher powered amplifier having linear characteristics in the presence of large amplitude input signals. While the linearity of a high power amplifier reduces the probability of generating narrowband interference, high power amplifiers also increase current consumption of the devices in which they are used. In the portable communications market, having numerous portable devices powered by batteries, an increase in amplifier power consumption leads to reduced battery life, which is of great concern to many consumers who desire not to be tethered by power cords. Accordingly, in an effort to eliminate narrowband interference, which may only be periodically present, battery life would always be sacrificed through the elimination of a low power LNA in favor of a high power amplifier having linear characteristics.
The two approaches for eliminating or reducing narrowband interference that are discussed above sometimes needlessly sacrifice signal-to-noise ratio or battery life when narrowband interference is not even present. Such tradeoffs are significant and would ideally be avoided altogether when attempting to eliminate or reduce narrowband interference in a communication receiver.
A receiver generates two metrics, one that is used to control the gain of an amplifier and the other that is used to determine the presence of narrowband interference, such as IMD. The two metrics may represent analog-to-digital converter (A/D) saturation and average signal strength, either of which may be used to control gain or to detect the presence of narrowband interference.
According to a first aspect, the present invention may be embodied in an automatic gain controller for use with a receiver adapted to receive a broadcast signal having digital information encoded therein. The receiver may include a first amplifier having a fixed gain setting and a unity gain setting, a second amplifier cascaded with the first amplifier, wherein the second amplifier has a variable gain, and an automatic gain controller. In such an arrangement, the automatic gain controller may include a signal strength determiner adapted to process the digital information to produce a signal strength metric representative of an average signal strength of the broadcast signal and a threshold comparator adapted to process the digital information to produce a threshold satisfaction indication representative of a magnitude of the digital information in comparison to a threshold. The automatic gain controller may also include an accumulator communicatively coupled to the threshold comparator and adapted to process the threshold satisfaction indication to determine a historical probability that the digital information exceeds the threshold to produce a threshold satisfaction metric and a gain adjuster that may be communicatively coupled to the signal strength determiner, the accumulator, the first amplifier and the second amplifier. The gain adjuster may be adapted to selectively switch the first amplifier between the fixed gain setting and the unity gain setting and to vary the gain of the second amplifier based on the signal strength metric and the threshold satisfaction metric.
The signal strength determiner may processes the digital information by summing a number of absolute values of analog signal levels represented by the digital information and dividing the sum by the number of absolute values over which the sum was taken or by summing a number of squared values of analog signal levels represented by the digital information and dividing the sum by the number of squared values over which the sum was taken to produce the signal strength metric. Alternatively, the digital information may be processed by summing a number of squared absolute values of analog signal levels represented by the digital information and dividing the sum by the number of squared absolute values over which the sum was taken to produce the signal strength metric.
The gain adjusted may be adapted to compare a current value of the signal strength metric with a prior value of the signal strength metric to determine whether the second amplifier should be set to the fixed gain setting. Further, the gain adjuster may be adapted to set the second amplifier to the fixed gain setting when the current value of the signal strength metric is substantially equal to the prior value of the signal strength metric. Alternatively, the gain adjuster may be adapted to set the first amplifier to the unity gain setting when the current value of the signal strength metric is not substantially equal to the prior value of the signal strength metric.
The prior value of the signal strength metric may be determined when the first amplifier is set to unity gain and the current value of the signal strength metric may be determined when the first amplifier is set to fixed gain. Alternatively, the prior value of the signal strength metric may be determined when the first amplifier is set to fixed gain and the current value of the signal strength metric may be determined when the first amplifier is set to unity gain.
In some embodiments, the gain adjuster may be adapted to compare a current value of the threshold satisfaction metric with a prior value of the threshold satisfaction metric to determine whether the second amplifier should be set to the fixed gain setting. In particular, the gain adjuster may be adapted to set the second amplifier to the fixed gain setting when the current value of the threshold satisfaction metric is substantially equal to the prior value of the threshold satisfaction metric. Alternatively, the gain adjuster may be adapted to set the first amplifier to the unity gain setting when the current value of the threshold satisfaction metric is not substantially equal to the prior value of the threshold satisfaction metric. Further, the prior value of the threshold satisfaction metric may be determined when the first amplifier is set to unity gain and the current value of the threshold satisfaction metric may be determined when the first amplifier is set to fixed gain. Additionally, the prior value of the threshold satisfaction metric may be determined when the first amplifier is set to fixed gain and the current value of the threshold satisfaction metric may be determined when the first amplifier is set to unity gain.
According to a second aspect, the present invention may be embodied in an automatic gain control method for use with a receiver adapted to receive a broadcast signal having digital information encoded therein, the receiver may include a first amplifier having a fixed gain setting and a unity gain setting and a second amplifier cascaded with the first amplifier, wherein the second amplifier may have a variable gain. The method may include the steps of producing a first metric representative of a signal strength of the broadcast signal, producing an indicator signal representative of a magnitude of the digital information in comparison to a threshold, processing the indicator signal to determine a historical probability that the digital information exceeds the threshold to produce a second metric representative of an average signal strength of the signal and selectively switching the first amplifier between the fixed gain setting and the unity gain setting and varying the gain of the second amplifier based on the first metric and the second metric.
According to a third aspect, the present invention may be embodied in an automatic gain controller for use with a receiver having a processor, wherein the receiver may be adapted to receive a broadcast signal having digital information encoded therein. The receiver may include a first amplifier having a fixed gain setting and a unity gain setting and a second amplifier cascaded with the first amplifier, wherein the second amplifier may have a variable gain. In such an arrangement the automatic gain controller may include a memory communicatively coupled to the processor, a first routine stored on the memory for causing the processor to process the digital information to produce a signal strength metric representative of an average signal strength of the broadcast signal, a second routine stored on the memory for causing the processor to process the digital information to produce a threshold satisfaction indication representative of a magnitude of the digital information in comparison to a threshold, a third routine stored on the memory for causing the processor to process the threshold satisfaction indication to determine a historical probability that the digital information exceeds the threshold to produce a threshold satisfaction metric and a fourth routine stored on the memory for causing the processor to selectively switch the first amplifier between the fixed gain setting and the unity gain setting and to vary the gain of the second amplifier based on the signal strength metric and the threshold satisfaction metric.
These and other features of the present invention will be apparent to those of ordinary skill in the art in view of the description of the preferred embodiments, which is made with reference to the drawings, a brief description of which is provided below.